Referring to FIG. lA, a deposition apparatus in accordance with the prior art for depositing a thin film layer to a surface of a workpiece, such as a semiconductor wafer is depicted. The apparatus is constructed of a combined cathode and target electrode assembly, including a cooling mechanism (not shown), which are generally designated by reference numeral 70. A power supply 71 is coupled to the assembly 70 for applying a high voltage thereto. A workpiece 72, such as a semiconductor wafer having a generally circular configuration, is placed on a mounting mechanism generally designated by reference numeral 75, which may rotate the workpiece 72 relative to the assembly 70. When voltage is applied to the assembly 70, deposition of a thin film layer from the target electrode occurs on the exposed surface of the workpiece 72. A housing 77, wherein a vacuum is maintained, contains the assembly 70, the workpiece 72, and the mounting mechanism 75.
Referring to FIG. lB, a portion of the cathode and target electrode assembly of the prior art is depicted in detail. A circular target electrode 11, as a source of material to be sputtered, is disposed adjacent a circular cathode 12. For assembly purposes, a slight gap exists between the cathode 12 and the target electrode 11 at a planar surface 22 of the target electrode, when the target electrode is cool. Pipes 13 are utilized to convey water or other cooling fluid to a chamber 20 provided adjacent the surface of the cathode 12, for cooling the cathode and primarily the target electrode 11. During the sputtering process, heat is generated at a surface 11A of the target electrode 11. In response, the target electrode 11 expands radially outward, pressing against a surface 15 of the cathode 12. Accordingly, cooling of the target electrode 11 by the water in the chamber 20 is effected primarily through the surface 15 of the cathode 12. Owing to the radial expansion of the target electrode 11 upon heating, the fit of the target electrode 11 and the cathode 12 at the surface 22 of the target electrode remains relatively loose, providing inefficient cooling of the target electrode 11 thereat.
Further, with radial expansion of the target electrode 11, contact of the target electrode 11 and the cathode 12 is reduced at a surface 25 of the cathode 12, limiting cooling therethrough. Thus, cooling of the target electrode 11 is afforded primarily via the surface 15 of the cathode 12. With heating occurring at the surface 11A of the target electrode 11, significant temperature differentials occur throughout the target material, across the gap at the surface 22 to the water cooled cathode 12, and through the cathode 12 itself.
At a power flux of about 160W/cm.sup.2 typically flowing across the surface 15 of the cathode 12, a temperature differential of approximately 100.degree. C. can exist. Owing to the length of the path through the cathode 12 from the surface 15 to the chamber 20, an additional drop of about 80.degree. C. can also exist. Assuming a reasonable flow and temperature of the cooling water inlet via the pipes 13, the coolest portion of the target electrode 11 at the surface 15 of the cathode 12 is generally at a temperature of at least 230.degree. C. From the surface 15 of the cathode 12 to the surface 11A of the target electrode 11 there can be a temperature rise in the case of aluminum alloys of an additional 250.degree. C., which is approximately the melting point of some eutectics which may be present in the target material. The generation of such high temperatures in the target electrode 11, particularly at the surface 11A, results in an unacceptable lack of control of the deposition process which can produce a runaway voltage at high power.
One known cathode and target electrode assembly which overcomes the above mentioned cooling problem incorporates a target electrode which is soldered directly within the hollow of the cathode to provide a unitary construction. This assembly, however, suffers from a number of notable drawbacks which results from this construction. The most significant one is the inability of this assembly to permit replacement of the target electrode in a simple and economical manner. For example, it is required that all remanents of the target electrode be completely machined from within the cathode hollow prior to soldering a replacement target electrode.
Accordingly, there is a need for a simple, efficacious means of cooling the target electrode 11 of such a deposition apparatus.